1. Field of the Invention
The present invention relates generally to a semiconductor memory device, and more particularly, to a nitride read only memory device with buried diffusion spacers and a method for making such a device.
2. Description of Related Art
As is well known in the art, a nitride read only memory device has an oxide-nitride-oxide (ONO) layer disposed over its silicon substrate. A polysilicon gate is disposed over the ONO layer. Two buried diffusion (BD) regions are implanted on the silicon substrate and are positioned adjacent to sidewalls of the polysilicon gate. Functioning as a source and a drain for the nitride read only memory device, the two BD regions are implanted through an ion implantation process. A channel is formed at the silicon substrate underneath the ONO layer and between the two BD regions.
During the fabrication process, the increase of thermal budget will improve the reliability of the nitride read only memory device. However, the increase of thermal budget can also make the nitride read only memory device to be prone to the short channel effect. In addition, the scaling of the nitride read only memory device can also induce the short channel effect. One way to prevent the occurrence of the short channel effect for this nitride read only memory device is to implant two pocket implant regions at each end of the channel with each pocket implant region adjacent to one of the BD regions. Nevertheless, because the two pocket implant regions are implanted at a tilt angle, the pocket implantation process will cause implant damages at the sidewalk of the polysilicon gate and the areas of the ONO layer that are close to where the charges are stored, which will jeopardize the reliability of the nitride read only memory device.
In view of the foregoing, there is a need for a nitride read only memory device and a fabrication method that will improve the reliability of the device and suppress the short channel effect.